Method for machining ceramic workpiece with composite vibration

ABSTRACT

A method for machining a ceramic workpiece includes providing a sonotrode that has a transducer and a horn arranged along an axis, and the horn has helical slots and terminates at a tip, bringing the tip into proximity of the ceramic workpiece and providing an abrasive media to a work zone around the tip, using the transducer to produce ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip, and the helical slots convert a portion of the axial vibration to torsional vibration at the tip, and the axial vibration and the torsional vibration causing the abrasive media to abrade the ceramic workpiece in the work zone and thereby remove a localized portion of the ceramic workpiece.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority to U.S. Provisional Application No. 63/281,156 filed Nov. 19, 2021.

BACKGROUND

Airfoils and other components in a turbine section of a gas turbine engine are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic materials, such as monolithic ceramics, ceramic matrix composites, and combinations of these, are under consideration to replace superalloys. Among other attractive properties, ceramic materials have high temperature resistance. Ceramic materials, however, typically cannot be directly substituted for a superalloy. Rather, there are manufacturing and design factors that are unique to ceramics and which challenge practical implementation.

SUMMARY

A method for machining a ceramic workpiece according to an example of the present disclosure includes providing a sonotrode that has a transducer and a horn arranged along an axis. The horn has helical slots and terminates at a tip. The tip is brought into proximity of the ceramic workpiece and an abrasive media is provided to a work zone around the tip. The transducer produces ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip. The helical slots convert a portion of the axial vibration to torsional vibration at the tip. The axial vibration and the torsional vibration cause the abrasive media to abrade the ceramic workpiece in the work zone and thereby remove a localized portion of the ceramic workpiece.

In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the second section.

In a further embodiment of any of the foregoing embodiments, the second section is cylindrical and has a solid core.

In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a constant depth, and a ratio of the diameter to the constant slot depth is 5:1 to 10:1.

In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a slot length, and a ratio of the diameter to the slot length is 1:1 to 1:4.

In a further embodiment of any of the foregoing embodiments, each of the helical slots has a constant depth and a slot length, and a ratio of the slot length to the constant slot depth is 5:1 to 20:1.

In a further embodiment of any of the foregoing embodiments, the second section has a diameter and each of the helical slots has a slot length and a constant slot depth, and a ratio of the slot length to the constant slot depth divided by the diameter is 1:1 to 1:2.

In a further embodiment of any of the foregoing embodiments, each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position.

In a further embodiment of any of the foregoing embodiments, the second common axial position is no more than 12.7 millimeters from the tip.

In a further embodiment of any of the foregoing embodiments, the first slot end and the second slot end are circumferentially offset by 45° to 135°.

In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the first section.

In a further embodiment of any of the foregoing embodiments, each of the helical slots defines an angle of 30° to 60° with the axis.

In a further embodiment of any of the foregoing embodiments, the horn is a step horn.

In a further embodiment of any of the foregoing embodiments, the ceramic workpiece is a ceramic matrix composite.

An ultrasonic machining system according to an example of the present disclosure includes a sonotrode that has a transducer and a horn arranged along an axis. The horn has helical slots and terminates at a tip. Upon operation, with the tip in proximity of a ceramic workpiece, the transducer produces ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip. The helical slots convert a portion of the axial vibration to torsional vibration at the tip, and the axial vibration and the torsional vibration cause an abrasive media in a work zone around the tip to abrade the ceramic workpiece and thereby remove a localized portion of the ceramic workpiece.

In a further embodiment of any of the foregoing embodiments, the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on either the first section or the second section.

In a further embodiment of any of the foregoing embodiments, the helical slots are on the second section, the second section has a diameter, each of the helical slots has a constant depth, each of the helical slots has a slot length, a ratio of the diameter to the constant slot depth is 5:1 to 10:1, and a ratio of the diameter to the slot length is 1:1 to 1:4.

In a further embodiment of any of the foregoing embodiments, a ratio of the slot length to the constant slot depth is 5:1 to 20:1.

In a further embodiment of any of the foregoing embodiments, each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position.

In a further embodiment of any of the foregoing embodiments, the second common axial position is no more than 12.7 millimeters from the tip, the first slot end and the second slot end are circumferentially offset by 45° to 135°, and each of the helical slots defines an angle of 30° to 60° with the axis.

The present disclosure may include any one or more of the individual features disclosed above and/or below alone or in any combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

FIG. 1 illustrates an example sonotrode for ultrasonic machining of ceramic material.

FIG. 2 illustrates a portion of the horn of the sonotrode.

FIG. 3 illustrates another example sonotrode.

FIG. 4 illustrates an ultrasonic machining system during operation to produce a hole in a ceramic workpiece.

DETAILED DESCRIPTION

One challenge to implementing ceramic materials in place of superalloys is that ceramic materials must be processed differently than superalloys. The processes used to form ceramic materials into the desired geometry of a functional component have unique limitations. For instance, for a superalloy, cooling holes, slots, and the like can be formed during casting or, for relatively small dimensions, by precision machining after casting. Ceramic materials, however, are hard and brittle in comparison to superalloys. As a result, there is considerable difficulty in efficiently machining holes, slots, or other small features, and doing so with a desired degree of accuracy. Ultrasonic machining (“USM”) is one technique that is under consideration for forming these features. USM generally involves mechanical vibration at approximately 20 kHz or more in the presence of an abrasive media to cause removal of material. When used on ceramics, however, USM yields low material removal rates that are insufficient for practical implementation on ceramics. In this regard, as will be discussed herein, the present disclosure provides a method and system for USM that facilitates increased material removal rates on ceramic materials.

FIG. 1 illustrates an example sonotrode 20 for facilitation of increased material removal rates in USM systems. The sonotrode 20 is operable to provide a composite axial-torsional vibrational mode in order to enhance material removal. The sonotrode 20 has a transducer 22 and a horn 24 that are generally arranged along a central axis (A). The transducer 22 may include one or more piezoelectric elements that, when activated with an electric current, produces vibrational waves that propagate axially (i.e., axial vibration V1).

The horn 24 is mechanically coupled to the transducer 22 and includes several sections. As shown, the horn 24 is a step horn, although it is to be understood that the type of horn is not necessarily limited to step horns. The horn 24 includes a first section 26 and a second section 28. A least a portion of the first section 26 tapers in cross-section, to focus the vibration. In the illustrated example, the initial portion of the first section 26 adjacent to the transducer 22 is cylindrical but then transitions to conical. The second section 28 has a uniform cross-section and terminates at a tip 30. In this example, the second section 28 is cylindrical. Both the first section 26 and the second section 28 are solid and may be formed from an alloy or steel, such as but not limited to an aluminum alloy or steel.

The horn 24 further includes helical slots 32. In this example, the helical slots 32 are on the second section 28. The helical slots 32 serve to convert a portion of the axial vibration (V1) to torsional vibration V2, while limiting excitation of undesirable bending modes. The degree and manner to which the helical slots 32 do this can be controlled via the slot geometry.

As shown in representative FIG. 2 , each slot 32 defines a first slot end 32 a that is distal from the tip 30 and a second slot end 32 b that is proximal to the tip 30. The first slot ends 32 a are located at a first common axial position A1, and the second slot ends 32 b are located at a second common axial position A2. Thus, as the axial vibration V1 propagates down the horn 24 it encounters, and is acted upon by, all of the slots 32 at once.

As also shown in FIG. 2 , the second section 28 has a diameter D, each of the helical slots 32 has a constant depth d, a slot length L, and a circumferential offset C. The depth d is the distance from the surface of the second section 28 to the floor of the slot 32. The slot length L is the linear axial distance from the first slot end 32 a to the second slot end 32 b, and the circumferential offset C is the length of the arc segment in degrees between the first end 32 a and the second end 32 b.

In one example, a ratio of the diameter D to the constant slot depth d is 5:1 to 10:1. In a further example, a ratio of the diameter D to the slot length L is 1:1 to 1:4. In a further example, a ratio of the slot length L to the constant slot depth d is 5:1 to 20:1. In a further example, a ratio of the slot length L to the constant slot depth d divided by the diameter D is 1:1 to 1:2. In a further example, each of the slots 32 has an angle G with respect to the axis A that is from 30° to 60°. In a further example of any of the above examples, the first slot end 32 a and the second slot end 32 b are circumferentially offset by 45° to 135°. In a further example of any of the above examples, the second common axial position A2 is also no more than 12.7 millimeters from the tip.

The sonotrode 20 with the above features, or combinations thereof, facilitates adaptation of USM for the machining of ceramic material. For instance, most of the material removal is due to the axial vibration V1. Therefore, the portion of the axial vibration V1 that is converted into the torsional vibration V2 can be limited via the above prescribed ranges. Moreover, the cycles of vibration should be in sync such that the peak amplitude of the axial vibration V1 coincides with the peak amplitude of torsional vibration V2. Also, the torsional vibration V2 can be primarily induced at or near the tip 30 by placing the slots 32 near the tip 30 per the above range. In one alternative shown in FIG. 3 , however, the helical slots 32 are located on the conical portion of the first section 26.

FIG. 4 illustrates an example of a USM system during operation to machine a ceramic workpiece 40. The ceramic material of the workpiece 40 is not particularly limited and may be a monolithic ceramic, a ceramic matrix composite (CMC), or combinations of monolithic and CMC. The monolithic ceramic may be, but is not limited to, silicon nitride or silicon carbide. The ceramic matrix composite may be, but is not limited to, a SiC/SiC ceramic matrix composite in which SiC fiber tows are disposed within a SiC matrix. Alternatively, the fibers and/or matrix may be Si₃N₄.

With the tip 30 in proximity of the ceramic workpiece 40, the transducer 22 (FIG. 1 ) produces ultrasonic vibration that axially propagates down the horn 24 and causes axial vibration V1 at the tip 30. The aforementioned helical slots 32 convert a portion of the axial vibration V1 to torsional vibration V2 at the tip 30. The axial vibration V1 and the torsional vibration V2 cause an abrasive media 42 containing abrasive particles 44 in a work zone Z around the tip 30 to abrade the ceramic workpiece 40 and thereby remove a localized portion of the ceramic workpiece 40. For instance, at the peak amplitude of the axial vibration the abrasive particles 44 are driven to penetrate into the exposed surface of the ceramic workpiece 40. Simultaneously, the torsional vibration acts to drive the abrasive particles 44 sideways across the exposed surface, causing the cutting off of “microchips” of ceramic and smoothing of the surface. The simultaneous penetration, cutting, and smoothing facilitates an increase in material removal rate and accuracy in comparison to using only axial vibration, thereby enabling more practical application of USM for ceramic material. During the remaining vibration cycle, the horn 24 and the ceramic workpiece 40 are separated and there is thus little material removal.

The tip 30 of the sonotrode 20 can be advanced into the ceramic workpiece 40 as material is removed in order to form a deeper hole and/or translated along the surface of the ceramic workpiece 40 to produce a slot. Additionally, a mass element 25 (FIG. 1 ) may be provided on the opposite axial side of the transducer 22 from the horn 24. The displacement the tip 30 is larger than at the back side of the transducer 22 because the mass element 25, which may be made from steel, is of relatively higher impedance than the horn 24 (which may be made from aluminum). This prevents the backward propagation of the axial vibration to improve the output amplitude at the tip 30.

Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. A method for machining a ceramic workpiece, the method comprising: providing a sonotrode that has a transducer and a horn arranged along an axis, and the horn has helical slots and terminates at a tip; bringing the tip into proximity of the ceramic workpiece and providing an abrasive media to a work zone around the tip; using the transducer to produce ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip, and the helical slots convert a portion of the axial vibration to torsional vibration at the tip; the axial vibration and the torsional vibration causing the abrasive media to abrade the ceramic workpiece in the work zone and thereby remove a localized portion of the ceramic workpiece.
 2. The method as recited in claim 1, wherein the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the second section.
 3. The method as recited in claim 2, wherein the second section is cylindrical and has a solid core.
 4. The method as recited in claim 3, wherein the second section has a diameter and each of the helical slots has a constant depth, and a ratio of the diameter to the constant slot depth is 5:1 to 10:1.
 5. The method as recited in claim 3, wherein the second section has a diameter and each of the helical slots has a slot length, and a ratio of the diameter to the slot length is 1:1 to 1:4.
 6. The method as recited in claim 3, wherein each of the helical slots has a constant depth and a slot length, and a ratio of the slot length to the constant slot depth is 5:1 to 20:1.
 7. The method as recited in claim 3, wherein the second section has a diameter and each of the helical slots has a slot length and a constant slot depth, and a ratio of the slot length to the constant slot depth divided by the diameter is 1:1 to 1:2.
 8. The method as recited in claim 1, wherein each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position.
 9. The method as recited in claim 7, wherein the second common axial position is no more than 12.7 millimeters from the tip.
 10. The method as recited in claim 7, wherein the first slot end and the second slot end are circumferentially offset by 45° to 135°.
 11. The method as recited in claim 1, wherein the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on the first section.
 12. The method as recited in claim 1, wherein each of the helical slots defines an angle of 30° to 60° with the axis.
 13. The method as recited in claim 1, wherein the horn is a step horn.
 14. The method as recited in claim 1, wherein the ceramic workpiece is a ceramic matrix composite.
 15. An ultrasonic machining system comprising: a sonotrode that has a transducer and a horn arranged along an axis, the horn having helical slots and terminating at a tip, wherein upon operation, with the tip in proximity of a ceramic workpiece, the transducer produces ultrasonic vibration that axially propagates down the horn and causes axial vibration at the tip, the helical slots convert a portion of the axial vibration to torsional vibration at the tip, and the axial vibration and the torsional vibration cause an abrasive media in a work zone around the tip to abrade the ceramic workpiece and thereby remove a localized portion of the ceramic workpiece.
 16. The ultrasonic machining system as recited in claim 15, wherein the horn includes a first section that tapers and a second section that has a uniform cross-section, and the helical slots are on either the first section or the second section.
 17. The ultrasonic machining system as recited in claim 16, wherein the helical slots are on the second section, the second section has a diameter, each of the helical slots has a constant depth, each of the helical slots has a slot length, a ratio of the diameter to the constant slot depth is 5:1 to 10:1, and a ratio of the diameter to the slot length is 1:1 to 1:4.
 18. The ultrasonic machining system as recited in claim 17, wherein a ratio of the slot length to the constant slot depth is 5:1 to 20:1.
 19. The ultrasonic machining system as recited in claim 15, wherein each of the helical slots defines a first slot end that is distal from the tip and a second slot end that is proximal to the tip, the first slot ends are located at a first common axial position, and the second slot ends are located at a second common axial position.
 20. The ultrasonic machining system as recited in claim 19, wherein the second common axial position is no more than 12.7 millimeters from the tip, the first slot end and the second slot end are circumferentially offset by 45° to 135°, and each of the helical slots defines an angle of 30° to 60° with the axis. 